The concept of the compound bow introduced mechanical advantages over the traditional straight and recurved bow designs whereby a system of cables, cams, and pulleys were interposed between the bowstring and the bow limbs to provide mechanical advantage in the draw and a property called let-off, that is, the force required to hold the bowstring at full draw length is substantially lower than that required to hold said bowstring at intermediate draw length. The force that propels the arrow at any instantaneous position of the bowstring after release is approximately equal to that force required to hold the bowstring stationary in that position, thus in a compound bow, the arrow is subjected to higher acceleration at an intermediate position during release than would have been the case with a traditional bow of the same holding force at full draw. Thus compound bows result in higher arrow velocity. Also, arrows can be shot from such bows with more accuracy, as the archer is subjected to lower stress while aiming at full draw than in the case of traditional bow designs.
Compound bows have changed little in their basic designs although improvements in the eccentric cams and cable arrangements have resulted in increased arrow velocity. Prior art compound bows typically contain a pair of cables that almost span the entire space between the limb tips in such a way that these cables cross over each other and would interfere with the patch of the arrow were it not for the presence of special means to hold the cables aside. One such special means involves the use of a cable guard comprising a rod attached to the bow handle riser, said rod being offset from the centerline of the limbs and positioned between the cables and said centerline, thus holding the cables aside to provide clearance between the cables and the arrow path.
Another such special means involves the use of dual-grooved cams or pulleys at the limb tips with one or both grooves offset from said centerline and arranged in such a way that the primary cable attached directly to the bowstring is disposed in a different groove from that in which is disposed the secondary cable attached to the opposite limb tip. The spacing between the grooves provides the necessary cable clearance to avoid interference with the arrow path.
Both of the above special means to avoid cable interference suffer a common disadvantage that can impact arrow flight accuracy. Any offsetting of the cables, cams, or bowstring from the centerline of the bow limbs can result in an imbalance in cable tensions and a corresponding torque or twisting of the limbs. The energy stored in such twisting is not only wasted as the arrow is released but the relaxation of the torque forces can impart minute sideways motion of the bowstring and consequently cause unstable arrow flight. An additional disadvantage of the crossing over of the cables is that they can touch each other causing chafing and also dissipate energy in the process of rubbing together.
It is thus a desirable feature in a compound bow design to align all cables, cams, and pulleys with respect to the centerline of the limbs in such a way as to balance the forces acting on said limbs to avoid torque or twisting moments. It is also desirable that the bowstring be accurately aligned with said centerline and further that the cables not come in contact with each other.